1. Field of Invention
The present invention relates in general to receivers and methods of receiving for communication systems and, in particular, to an optical signal receiver and optical signal receiving method including decision threshold adjustment.
2. Description of Related Art
It is well known that signals transported over optical communication networks suffer degradation between associated transmitters and receivers. Signal degradation may result from a variety of system parameters including the total transmission distance, the transmission fiber type, the number of optical amplifications to a signal, the number of system channels, etc.
Optical networks are, however, being developed with ever-increasing signal transmission speeds and distances. Channel counts have also been increasing in wavelength division multiplexed transmission systems. The greater transmission distances, speeds and higher channel counts directly effect received signal quality.
To maintain high fidelity signal reception in optical networks, advances in receiver design have been proposed. For example, receivers are constructed with the goal of achieving an acceptable BER (bit error rate), which is the ratio of the number of incorrectly received bits to the total number of received bits. Typically, this is achieved by adjusting and fixing the decision threshold of a comparator within the receiver while providing a well-known optical test signal at the comparator data input. The decision threshold is a reference voltage against which the strength of a received signal is compared. If the received signal is above the decision threshold, it is interpreted as being “on”, but if the received signal is below the decision threshold, it is interpreted as being “off”.
It is also known that a decision threshold may be established from the eye diagram of the received signal. An exemplary eye diagram is illustrated in FIG. 1a which is also useful for understanding the basic concept of a decision threshold for an optical signal (aka “slicing level”).
In general, an eye diagram may be observed on an oscilloscope by monitoring the receiver data output voltage on the vertical input of the oscilloscope and triggering on the data clock. Key features of an eye diagram, as illustrated in FIG. 1a, include the crossing points C1, C2, useable eye width (i.e. the time distance on the horizontal scale between points C1 and C2) and usable eye height H (voltage).
In an ideal received signal such as the one shown in FIG. 1a, the crossing points C1,C2 would be centered and symmetrical, and the open area would be as large as possible, indicating strong pulse width and height in the received signal. A decision threshold DT may be selected to correspond with the voltage level at the crossing points C1, C2, as shown.
A difficulty with such receiver designs, however, is that the eye diagram itself depends on all of the above-mentioned parameters that effect signal quality. All these parameters can change with time, e.g. due to system upgrades to add more channels, changes in fiber plant, or changes in optical amplifier performance. Such changes can alter the received “eye diagram” leading to a degradation of the BER in the absence of a re-optimized decision threshold.
In an attempt to minimize the adverse effects of system changes and generally increase effective transmission distances through greater error tolerance, error correction schemes such as forward error correction (FEC) have been incorporated into receiver designs. FEC generally includes generation of a control code at the transmission site. The control code is transmitted with the data to a receiver. Error correction may be achieved based on various algorithms that compensate for specific detected errors in the control code. Although FEC schemes have achieved wide acceptance, there is room for improvement in basic receiver design that addresses the underlying BER variation resulting from changes in system parameters.
In amplified optical transmission systems such as DWDM (dense wave division multiplexed) rings, many channels share the same optical amplifier even though each channel may traverse different optical paths and have different source and termination points. Such a multi-channel optical amplifier is quite well known (e.g. an erbium doped fiber amplifier) and is considered an enabling technology of WDM systems. One problem is that channels passing through the same optical amplifier must share the same gain medium of the optical amplifier. Changes in the signal level of one or more channels going through the amplifier impact the other channels to some degree since they share the same gain medium.
In order to limit the impact on the other channels, optical amplifiers can be designed and operated in various modes. One of the well-known techniques is to operate the amplifier in gain control mode so that the gain of the amplifier is kept constant regardless of total input power coming in to the amplifier. In this way, the output power for other channels not directly impacted by input power level changes can be kept constant. This concept can be best described with reference to FIGS. 1b-g. 
As shown in FIG. 1b, the optical amplifier (amp) in the NE2 (network element 2) is shared by two optical channels, one channel going from NE1 to NE3 and second channel being optically muxed going from NE2 to NE3. In other words, separate transmitters (Tx1 and Tx2) inject separate optical channels into different locations of the optical fiber of the network as is well-known in the art.
FIGS. 1c and 1d show the average optical receiver power seen by two receivers (Rx1 and Rx2, respectively) when the network is in steady state.
FIG. 1e illustrates the same network as FIG. 1b except that there is a failure in channel 1 (indicated by a large X between NE1 and NE2). The illustration is most indicative of a fiber cut but there are a variety of circumstances that can lead to the failure of a subset of the channels being amplified by a common amplifier. In terms of channel 1 failing, these circumstances include, but are not limited to, laser on Tx 1 malfunctions, someone mistakenly either removes the circuit pack containing Tx 1 from the chassis, somebody removes a fiber patch cord connected to the Tx 1, the amplifier on the NE1 either malfunctions or is removed by mistake (or removed for some other purpose such as upgrade or maintenance), a power outage on the NE1 causes the Tx 1 and/or amplifier to shut down, a fiber cut between NE1 and NE2. Any of the above events will cause the channel 1 power to fall to zero. Thus, the amplifier in NE2 will also suddenly see half as much power (channel 1 now zero, channel 2 still present).
In other words, a fiber cut between NE1 and NE2 causes the power of channel 1 to drop to zero. As a result, the received power level of channel 1 also drops close to zero (see FIG. 1f). If the amplifier located at NE2 is in a gain mode (as it normally would be), when the power of channel 1 disappears there is extra energy available which causes the gain of the channel 2 to go up for a short amount of time at least until conventional optical and electrical control loops in the amplifier can react to bring the gain to the same level as before for channel 2. One can improve the time scale necessary to remove the extra energy available from the amplifier by designing faster amplifier control loops; however, it can't be completely eliminated. Commercially-available, state-of-the-art amplifiers can react to the changes in channel 1 provided the changes occur with time scale that is much larger than few hundred microseconds. However, some of the events described above can cause the channel 1 to drop within 20 to 50 microseconds. As shown in FIG. 1g, the average received power of channel 2 (as seen by receiver Rx2) exhibits a rapid increase followed by rapid decrease to its steady state value. Such a large transient is quite typical of conventional optical networks.
Moreover, there is a snowball effect in optical networks due to the amplifier chain. The transient signal (both the amplitude and speed of the change) increases as more and more amplifiers are cascaded. In other words, the transient snowballs as it is successively amplified by a chain of amplifiers (e.g. between NE2 and receiver in the example discussed above).
In the example discussed above in relation to FIGS. 1b-g, a customer receiver that is receiving channel 2 (going between NE2 and NE3) should not be negatively affected when another channel in the network (e.g. channel 1 in the example) fails. Nevertheless, such a traffic impact may very well occur as these kind of network events cause the receiver power as well as optical signal to change rapidly at the receiver.
Moreover, optical networks are becoming more and more dynamic in their traffic configurations. For example, various forms of protection switching exist in order to compensate for problems such as a fiber break or equipment failure. As is known, protection switching may reroute the optical signal along an alternative path. In addition, optical signal traffic may need to be rerouted due to other concerns such as equipment maintenance, upgrades, and traffic load balancing. Both protection switching and signal rerouting cause what is variously referred to herein as a “channel-disrupting transient” or alarm condition that directly affects the channel being switched or rerouted.
The result of such protection switching and traffic rerouting is that the optical signal suddenly traverses a much longer (or perhaps shorter) distance and experiences a much greater (smaller) attenuation and distortion than before. In other words, after the receiver loses the signal, it will be restored along a different path with a different OSNR (optical signal to noise ratio), received power, and eye quality. The receiver must be able to adjust to those differences and detect the protection signal within the time scale dictated by the standards of the transmitted signal. Standards such as SONET and SDH provide for a 50 ms recovery time for a protection switch event. Thus, there is a time window of 50 ms for the traffic to be rerouted and for the system to recover to the point where the rerouted signal may be received with an acceptable BER.
While such a protection switch recovery time is quite useful for the channel being rerouted, it does not help the other unswitched channels. As explained above, the sudden absence of one channel (e.g. Channel 1) in an amplifier simultaneously amplifying plural channels (e.g. Channels 1 and 2) will cause a non-channel-disruptive transient in the other channels (Channel 2 in the example). Channel 2 is provided no such recovery time and the receiver for channel 2 must continue to receiver Channel 2 with an acceptable bit error rate. Thus, there is a need in the art for an improved optical receiver to handle situations like those faced by Channel 2 (e.g. in general terms, a transient caused by a sudden change in different channel that causes all commonly amplified channels to experience a transient).
Moreover, conventional systems utilize a fixed threshold that does not change in response to a large transient such as that resulting from a protection switch event. The fixed threshold is chosen to have sufficient margin to guarantee end-of-life performance (e.g. from laser aging). Such a fixed threshold is essentially a poor compromise for optimal performance and has significant impacts on the link budget thereby making it very difficult for high bit rate channels to be accommodated (nonlinear optical signal distortions typically increase with bit rate).
Accordingly, there is a need in the art for optical receiver configuration and/or optical signal control method that adjusts the receiver decision threshold to reduce the BER. In addition, there is a need in the art of adjusting the receiver decision threshold in a way that can adapt to both fast and slow network changes and achieve an acceptable BER within these different time scales.